The objective will be to determine how the quantities of two major nutrients, protein and lipid, interact to influence prolactin in male rats. Semipurified casein-based diets (2 times 2 factorial design) varying in fat (10 percent or 40 percent of calories) and protein (5 percent or 16 percent of calories) will be used in all studies. The proportion of protein, minerals, vitamins, and fiber will be adjusted to maintain a constant ratio with respect to calories. We will determine the main effects of dietary protein and fat as well as their interactions on 1) plasma and pituitary prolactin concentrations, 2) the secretory and metabolic clearance rates of prolactin, 3) the circulating forms of prolactin and their immunoreactivities, and 4) prolactin binding in the liver and prostate. The studies will be conducted at different stages of growth following assignment of the animals to the different dietary treatments.